Method for generating a corrected error signal, and corresponding apparatus

ABSTRACT

In order to obtain a corrected or compensated focus error signal or track error signal, it is proposed to generate primary and secondary scanning beams incident on adjacent tracks of an optical recording medium and to detect the primary and secondary scanning beams reflected from the optical recording medium in order to derive from them primary-beam and secondary-beam focus error signals or primary-beam and secondary-beam track error signals, which are subsequently normalized in order to obtain the compensated focus error signal or track error signal from the normalized primary-beam and secondary-beam error signals by means of weighted combinations. As a result of the normalization, the corrected or compensated focus error signal or track error signal is generated independently of the reflection properties of the respectively scanned track.

[0001] The present invention relates to a method for generating acorrected error signal, in particular an offset-compensated focus errorsignal or track error signal, for an apparatus for reading from and/orwriting to an optical recording medium according to the preamble ofclaim 1, and also to a correspondingly configured apparatus according tothe preamble of claim 16.

[0002] One of the widespread methods for forming a focus error signal isthe so-called astigmatism method. This method can be used if only onetype of track is intended to be scanned or there is minor interactionbetween the type of track region scanned and the focus error signal.Optical storage media in which information tracks are contained both indepressions, referred to as “groove”, and in elevations, referred to“land”, have different focus offsets during the focus error signalgeneration according to the traditional astigmatism method during thescanning of information tracks in tracks with depressions and trackswith elevations. The asymmetries of the track geometry (width ratios,flank slope of the track edges, etc) may be regarded as a reason forthis.

[0003] The problems associated with the conventional method will beexplained in more detail below.

[0004] The focus error signal is conventionally generated for exampleaccording to the DFE method (“Differential Focus Error”). When the DFEmethod is employed, the laser beam of an optical scanner comprises threebeams, namely a primary beam and two secondary beams which scan adjacenttracks of the respective optical storage medium or optical recordingmedium. The primary and secondary beams reflected from the opticalrecording medium are evaluated in order to obtain, in a manner dependentthereon, primary-beam and secondary-beam focus error signals from whichthe desired focus error signal is generated by means of a weightedcombination. In order to achieve the splitting into three beams, anoptical grating is inserted into the beam path of the light source.

[0005]FIG. 5 illustrates a corresponding arrangement. The light emittedby a light source or a laser 1 passes through a collimator lens 2 and isthen split into the primary beam (i.e. a 0 th-order beam) and the twosecondary beams (i.e. ±1 st-order beams) by a diffraction grating 3. Theprimary beam, which reads the information to be scanned in a track of acorresponding optical recording medium 7, usually contains the majority(approximately 80-90%) of the light information. The two secondary beamseach contain the remaining approximately 5-10% of the total lightintensity, it being assumed for the sake of simplicity that the lightenergy of the higher orders of diffraction of the grating 3 are zero.These three beams are focussed onto the optical recording medium 7 via apolarizing beam splitter 4 and a quarter-wave plate 5 and also anobjective lens 6, in order to read from and/or write to the said opticalrecording medium. The three beams reflected from the optical recordingmedium 7 are fed via the beam splitter 4 and a cylindrical lens 8 to aphotodetector unit 9, which detects the three beams reflected from theoptical recording medium 7. Connected to the photodetector unit 9 is anevaluation circuit 16 which evaluates the detected reflected primary andsecondary beams for the purpose of generating a focus error signal.Primary and secondary beams are spatially separate from one another onlyin the focussed or virtually focussed state, so that they areillustrated as a common beam pencil in the figure.

[0006] As is illustrated in FIG. 6 using a DVD-RAM as optical recordingmedium 7, the optical grating 3 is constructed in such a way that theimaging of the two secondary beams 13 and 15 scans precisely the centreof the secondary tracks or (in the case of media which can be written toonly in “groove” tracks) the centre beside the track scanned by theprimary beam 14. FIG. 6 also illustrates an example of the scanning ofoptical recording media, for example a CD-ROM or DVD-ROM, havingso-called “pits” 50, the direction of rotation of the optical recordingmedium being indicated in each case by the arrow in FIG. 6. Only a verysmall detail from the information-carrying layer of the recording medium7 is illustrated in each case in a diagrammatic illustration. The tracksdesigned as depressions are designated by “groove” or Gr and simplyshown hatched, while the tracks designed as elevations are designated by“land” or La and are not hatched. In the right-hand part of FIG. 6, thetracks provided with information are provided with diagrammaticallyillustrated pits, i.e. depressions or markings which influence a beamproperty in some other way.

[0007] Since the secondary beams 13 and 15 and the primary beam 14 areintended to be optically separable from one another, the positions oftheir imaging on the optical recording medium 7 and on the photodetectorunit 9 are separate from one another. If the optical recording medium 7rotates, then one of the secondary beams is situated in front of, andthe other secondary beam behind, the primary beam in the reading orwriting direction.

[0008] Considered by themselves in each case, both the primary beam andthe secondary beams generate, on the correspondingly chosenphotodetector unit 9 and after subsequent suitable combination of thedetector signals, a primary-beam and, respectively, secondary-beam focuserror signal which represents the focus error of the respective beamwith respect to the scanned surface of the optical recording medium 7.However, since the two secondary beams scan the two secondary trackswith respect to the actual read/write track (and hence the invertedposition “groove”/“land”), the focus offset error of the secondary beamsis inverted relative to the focus offset error of the primary beam.Consequently, considered by themselves, the respective focus errorsignals in each case contain the actual focus error with respect to theilluminated surface and also oppositely orientedtrack-position-dependent focus offset components.

[0009] In order to illustrate these facts, FIGS. 8A and 8B illustratethe detection of the primary and secondary beams reflected from theoptical recording medium 7 using the example of a photodetector unit 9,having three multi-zone photodetectors 10-12, the two photodetectors 10and 12 each being provided for detection of a secondary beam, while thephotodetector 11 serves for detection of the reflected primary beams.Each photodetector 10-12 has four photodetector elements, designated byE-G, A-D and I-L, respectively. This designation will also be usedhereinafter for referring to the output signals generated by thecorresponding photodetector elements. FIG. 8A illustrates the example ofa photodetector image given the presence of a track-position-dependentfocus offset component without focus error, while FIG. 8B illustratesthe example of a photodetector image with focus error but withouttrack-position-dependent offset.

[0010] If the focus error signals of the secondary beams are then addedand this sum is in turn added to the focus error signal of the primarybeam, these undesirable focus offset components cancel one another outgiven appropriate weighting between the primary and secondary beamcomponents. Since the focus error components of primary and secondarybeams are synchronous relative to one another, these are added in thecorrect phase.

[0011] Consequently, given correct setting of the weighting factor, allthat remains is the actual focus error without atrack-position-dependent focus offset component.

[0012] In this case, however, it must be taken into account that theamplitude of the focus error contribution of each scanning beam isproportional to the average reflection of the respectively scanned trackof the optical recording medium. Therefore, the functioning of thepreviously described procedure presupposes that the intensity ratiosbetween the primary beam and the secondary beams do not change relativeto one another, in order to be able to set a specific weighting factorfor compensation purposes. Herein lies the problem of the previouslydescribed method, however. If an optical recording medium that has beenwritten to completely is read, then the reflection properties of the“groove” and “land” tracks (for example in a DVD-RAM) are identical. Itcan then be assumed, in a simplification, that the undesirabletrack-position-dependent offset components and also the desired focuserror components of the three scanning beams each have the samemagnitude. Presupposing this, it is possible to find a weighting factorwhich provides for complete compensation of the track-position-dependentfocus offset components.

[0013] If, however, as is shown in FIG. 7, a hitherto blank opticalrecording medium, or one which has been recorded on only in part, iswritten to, then the primary beam used for writing alters the reflectionproperties of the storage medium on the tracks currently written to. Ifa “groove” track is written to in a DVD-RAM disc, for example, then onlythe reflection property of this track changes in the course of writing.The reflection properties of the “land” secondary tracks remainunaltered. This means that the weighting factor used hitherto no longerleads to compensation of the track-position-dependent focus offsetcomponents. The weighting factor is likewise no longer valid for a casein which the primary beam scans a written-to track and one of the twosecondary tracks has been written to but the other has not. Since thesectors of a DVD-RAM disc do not have to be written to continuously,problems can thus already arise during the scanning of such a disc ifthe reflection properties of the track read and the secondary tracksdiffer from one another. In order to illustrate this problem, FIG. 7illustrates the unwritten parts of the “groove” or “land” tracks, whichhave different reflection properties from the written parts, hatchedfrom top left to bottom right. The groove tracks, which are identifiedby hatching anyway, are thus doubly hatched in the written region, whilethe land tracks are not hatched in the unwritten region.

[0014] These problems can likewise occur in storage media on whichinformation is stored only in “groove” tracks. When a storage medium ofthis type is written to, the reflection of the written track likewisechanges. Since only “groove” tracks are written to, in theory thereflection properties of the regions between the tracks do not change asa result of the “groove” track being written to. In practice, however,owing to the small track spacings on the storage medium, there are alsoinfluences of the writing primary beam on the reflection of the regionsdirectly beside the primary track. This means that the beam leading tothe primary beam and the beam following the primary beam scan regionsbetween the tracks which have different reflection properties.

[0015] Thus, in the case of the two types of optical recording mediadescribed above, it is not possible to set a generally valid weightingfactor which enables complete compensation of thetrack-position-dependent focus offset component. Although in theory itis possible to determine and store different settings for the weightingfactor on the basis of the different properties of the respectivelyscanned track regions, it would then be necessary, with correspondingcomplexity, at all times to determine the state of the instantaneouslyscanned region and to set the appropriate weighting factor. However, inparticular in the case of track jumps and imperfections of the storagemedium to be read from or written to, this can lead to insolubleproblems since the currently scanned region of the storage medium thencannot be reliably ascertained.

[0016] A similar problem occurs when generating a track error signal.

[0017] The track error signal is conventionally generated for exampleaccording to the so-called DPP method (“Differential Push-Pull”), as isdescribed for example in the publication “Land/Groove Signal andDifferential Push-Pull Signal Detection for Optical Disks by an Improved3-Beam Method”, Ryuichi Katayama et al., Japanese Journal of AppliedPhysics, vol 38 (1999), pages 1761-1767. When the DPP tracking method isemployed, too, the original laser beam is split into three beams, namelya primary beam and two secondary beams which scan adjacent tracks of theoptical recording medium respectively used. As shown in FIG. 5, theprimary and secondary beams reflected from the optical recording mediumare detected by a photodetector unit 9 and evaluated by an evaluationcircuit 16 in order to obtain the track error signal. In the process,considered by themselves in each case, both the primary beam and thesecondary beams generate a push-pull signal which represents the trackerror of the respective signal with regard to the respectively scannedtrack. However, since the two secondary beams scan the secondary trackswith respect to the read/write track, their push-pull error is invertedwith respect to that of the primary beam. Considered by themselves, therespective push-pull components thus contain the actual track error withrespect to the respectively scanned track. Since the track position ofthe three beams can only change together, the three push-pull signalschange equally.

[0018] If the objective lens 6 is then moved in the track direction, theimaging of primary and secondary beams on the photodetector unit 9 alsomoves. This displacement of the imaging results in an offset voltage atthe output of the photodetector unit 9. The direction of this offsetvoltage is identical for all of the beams. The displacement of theobjective lens 6 thus gives rise to an offset voltage which does notoriginate from an actual track error and is therefore an interference.The genuine track error component and the undesirablelens-movement-dependent component are added in the push-pull signalyielded by the respective detectors of the photodetector unit 9.

[0019] For illustration purposes, FIG. 10A illustrates a photodetectorimage with push-pull, while FIG. 10B illustrates a photodetector imagewith spot movement. In both illustrations, it is assumed that thephotodetector unit 9 has three photodetectors 10-12, the photodetector11 detecting the primary beam reflected from the optical recordingmedium 7, while the other two photodetectors 10 and 12 detect thereflected secondary beams. Furthermore, it is assumed that thephotodetector is a four-quadrant detector (also cf. FIG. 8), while thetwo photodetectors 10 and 12, which serve for detection of the reflectedsecondary beams, merely have two photodetector elements E1 and E2, and,respectively, F1 and F2.

[0020] If the signals yielded by the photodetectors 10 and 12 are thenadded and this summation signal is subtracted from the signal of thephotodetector 11 which detects the reflected primary beam, then thepreviously described lens-movement-dependent component is cancelled outgiven appropriate weighting between the primary and secondary beamcomponents. However, since the push-pull components of primary andsecondary beams are inverted with respect to one another, they are addedin the correct phase after application of the subtraction. Consequently,given correct setting of the weighting factor, all that remains is theactual track error.

[0021] The previously described procedure for determining a corrected orcompensated track error signal is thus similar to the abovementionedprocedure for determining a compensated or corrected focus error signal.In the case of determining the corrected track error signal, too,however, the functioning of this method presupposes that the intensityratios between primary beams and secondary beams do not change relativeto one another.

[0022] If an optical storage medium that has been written to completelyis read, then the reflection properties of the “groove” tracks or “land”tracks are identical in the case of a DVD-RAM recording medium.Consequently, it is possible to find a weighting factor which providesfor complete compensation of the lens-movement-dependent components.

[0023] If, however, a hitherto blank optical medium is written to, thenthe primary beam used for writing alters the reflection properties ofthe optical storage medium on the tracks currently written to. If a“groove” track is written to on a DVD-RAM disc, for example, then onlythe reflection property of this track changes in the course of writing.The reflection properties of the “land” secondary tracks remainunaltered. This means that the weighting factor used hitherto no longerleads to the compensation of the lens-movement-dependent component. Theweighting factor is likewise no longer valid if the primary beam scans atrack that has already been written to, and one of the two secondarytracks has been written to, while the other has still not been writtento. Since the sectors of a DVD-RAM disc, for example, do not have to bewritten to continuously, problems can thus already arise during thescanning of such a disc if the reflection properties of the track readand of the secondary tracks differ from one another.

[0024] These problems can likewise occur in the case of storage media inwhich information is stored only in “groove” tracks. When a storagemedium of this type is written to, the reflection property of thewritten-to track likewise changes. Since only “groove” tracks arewritten to, in theory the reflection properties of the regions betweenthe tracks do not change as a result of the “groove” track being writtento. In practice, owing to the small track spacings on the opticalrecording medium, there are also influences of the writing primary beamon the reflection of the regions situated directly beside the primarytrack. This means that the secondary beam leading the primary beam andthe secondary beam following the primary beam read different reflectionproperties of the regions between the individual tracks.

[0025] Consequently, in the case of the two types of optical recordingmedia described previously, it is not possible to set a generally validweighting factor which achieves complete compensation of thelens-movement-dependent component.

[0026] EP 0 788 098 Al proposes generating a track error signal or afocus error signal from the output signals of a photodetector havingmulti-zone detector elements, the track error signal or focus errorsignal, after being generated, being divided by a summation signalcomposed, inter alia, of all the output signals of the individualphotodetector elements. In this way, normalization of the focus errorsignal or track error signal is carried out after the generationthereof.

[0027] The present invention is based on the object of proposing amethod for generating a corrected error signal for the operation of anapparatus for reading from and/or writing to an optical recordingmedium, and also a correspondingly configured apparatus, the compensatedor corrected error signal being obtained independently of the type andreflection property of the respectively scanned track of the opticalrecording medium.

[0028] This object is achieved according to the invention by means of amethod having the features of claim 1 and an apparatus having thefeatures of claim 16. The subclaims each define preferred andadvantageous embodiments of the present invention.

[0029] According to the invention it is proposed, for the purpose ofgenerating the respective error signal, which may be, in particular, afocus error signal or a track error signal, to generate primary andsecondary scanning beams which scan adjacent tracks of the opticalrecording medium respectively used. From the reflected primary andsecondary scanning beams, primary-beam and secondary-beam error signalsare respectively derived and normalized, the corrected or compensatederror signal being obtained from the normalized primary-beam andsecondary-beam error signals by means of weighted combination.

[0030] The error signals may be, in particular, focus error signalsobtained according to the DFE method, or track error signals obtainedaccording to the DPP method.

[0031] According to one variant of the invention, the primary-beam andsecondary-beam error signals are normalized separately in each case. Inaccordance with another variant, joint normalization is provided for thesecondary-beam error signals. For both cases, according to theinvention, exemplary embodiments are proposed which, by means of acorresponding choice of weighting factor, enable complete compensationof the track-position-dependent focus offset components (when generatingthe focus error signal) or of the lens-movement-dependent track offsetcomponent (when generating the track error signal) even in the event ofvarying reflection conditions of the respectively scanned tracks. Inthis way, stable and offset-free focus or track regulation is possibleindependently of the reflection conditions of the respectively scannedoptical recording medium, the normalization proposed according to theinvention also obviating the need to determine the weighting factorcontinuously using a suitable method.

[0032] Consequently, the present invention is a refinement of theso-called DFE or DPP methods for forming an offset-compensated focuserror signal or track error signal, respectively, with the presentinvention also being able to be applied in particular to opticalrecording media whose information tracks have been recorded on in partand are blank in part. In particular, the present invention can also beapplied to optical recording media whose information is stored both indepressions, i.e. in “groove” tracks, and in elevations, i.e. in “land”tracks, such as DVD-RAM discs for example.

[0033] The present invention is explained in more detail below usingpreferred exemplary embodiments with reference to the accompanyingdrawing. In this case, it is understood that modifications within thescope of expert ability likewise lie within the scope of the presentinvention.

[0034]FIG. 1 shows a first exemplary embodiment of the invention forgenerating an offset-compensated focus error signal,

[0035]FIG. 2 shows a second exemplary embodiment of the invention forgenerating an offset-compensated focus error signal,

[0036]FIG. 3 shows a third exemplary embodiment of the invention forgenerating an offset-compensated focus error signal,

[0037]FIG. 4 shows a fourth exemplary embodiment of the invention forgenerating an offset-compensated focus error signal,

[0038]FIG. 5 shows a simplified construction of an optical scanner forcarrying out the DFE method or DPP method according to the prior art,with this construction also being able to be applied to the presentinvention,

[0039]FIG. 6 and FIG. 7 show illustrations for illustrating the scanningof adjacent tracks of an optical recording medium by a primary beam andtwo secondary beams,

[0040]FIG. 8A shows a photodetector image upon application of the DFEmethod with the occurrence of a track-positioned-dependent focus offsetcomponent, but without the occurrence of an actual focus error,

[0041]FIG. 8B shows a photodetector image upon application of the DFEmethod with the occurrence of a focus error, but without the occurrenceof a track-position-dependent focus offset component,

[0042]FIG. 9 shows a circuit arrangement according to the prior art forgenerating an offset-compensated focus error signal,

[0043]FIG. 10A shows a photodetector image upon application of the DPPmethod with the occurrence of an actual track error, but without theoccurrence of a lens-movement-dependent track offset component,

[0044]FIG. 10B shows the illustration of a photodetector image uponapplication of the DPP method with the occurrence of alens-movement-dependent track offset component, but without theoccurrence of an actual track error,

[0045]FIG. 11 shows a circuit arrangement according to the prior art forgenerating an offset-compensated track error signal,

[0046]FIG. 12 shows a fifth exemplary embodiment of the invention forgenerating an offset-compensated focus and track error signal, and

[0047]FIG. 13 shows a sixth exemplary embodiment of the invention forgenerating an offset-compensated focus and track error signal.

[0048] As has already been mentioned previously, the focus error signalgenerated in accordance with the DFE method is composed, in practice, ofthe actual focus error and a track-position-dependent focus offsetcomponent. In order to generate an offset-compensated focus error signalDFE, the focus error signal CFE (“centre focus”) generated in a mannerdependent on the reflected primary beam is combined with the focus errorsignal OFE (“outer focus”) generated in a manner dependent on thereflected secondary beams, in a weighted manner as follows:

DFE=CFE+g*OFE  (1)

[0049] In this case, upon application of the photodetector structurehaving three four-quadrant detectors 10-12 which is shown in FIG. 8, thefollowing relationships hold true for the CFE signal and the OFE signal:

CFE=H′*((A+C)−(B+D))  (2)

OFE=L′*((E+G)−(F+H))+R′*((I+K)−(J+L))  (3)

[0050] In this case, g denotes the weighting factor, H′ denotes thereflection factor of the track scanned by the primary beam, and L′ andR′ denote the reflection factors of the secondary tracks which arescanned by secondary beams and run to the left and right alongside theprimary track scanned by the primary beam. A-L denote the output signalsof the photodetector elements of the individual photodetectors 10-12which are illustrated in FIG. 8.

[0051] The positions of the secondary beams on the optical recordingmedium are chosen in such a way that the track-position-dependent focusoffset components of the CFE signal and of the OFE signal are inantiphase. This is achieved, in theory, when the focus points of thesecondary beams lie on the centre of the complementary track withrespect to the track centre illuminated by the primary beam. If theweighting factor g is chosen correctly, then thetrack-position-dependent offset components of the primary beam and ofthe secondary beams cancel one another out after summation.

[0052] The actual deviations of the position of the objective lens withrespect to the information layer of the optical recording medium willaffect all three scanning beams equally. The resultant actual focuserror signals of the CFE signal and of the OFE signal are therefore inphase and are added.

[0053] The weighting factor g can be defined only if the reflectionfactors H′, L′ and R′ are identical or constant. However, as has beendescribed previously, this is not always ensured. The inventiontherefore proposes normalizing the reflection factors in order toachieve independence of the weighting factor g for the differentreflection properties of the scanned tracks. Since both the actual focuserror signal and the track-position-dependent focus offset componentsare proportional to the reflection of the respectively scanned track, itis greatly advantageous to achieve independence from the instantaneousreflection of the respectively scanned track by means of normalizationof the focus error components respectively generated by the threescanning beams.

[0054] The reflection factor H′ of the primary beam is proportional tothe total quantity of light which strikes the photodetector 11 havingthe photodetector elements A-D. Division by the summation signal of theindividual photodetector elements makes it possible to achievenormalization for the primary beam:

CFEN=((A+C)−(B+D))/(A+B+C+D)  (4)

[0055] In this case, CFEN denotes the normalized CFE signal. Thesummation signal (A+B+C+D) is proportional to the reflection factor H′of the primary beam. The same also applies correspondingly to thereflection factors L′ and R′, i.e. the reflection factor L′ isproportional to the summation signal (E+F+G+H) of the individualphotodetector elements of the photodetector 10, while R′ is proportionalto the summation signal (I+J+K+L) of the individual photodetectorelements of the photodetector 12. Consequently, it is possible to definea normalized OFE signal OFEN as follows:

OFEN=((E+G)−(F+H))/(E+F+G+H)+((I+K)−(J+L))/(I+J+K+L)  (5)

[0056] From this there results a first exemplary embodiment for acorresponding circuit arrangement in which normalization is providedseparately for each focus error signal derived from the three primaryand secondary beams, i.e. three dividers are required in order tonormalize the respective components of the three beams, the followingrelationship following from the formulae (1) and (4), (5) for thenormalized focus error signal DFEN:

DFEN=((A+C)−(B+D))/(A+B+C+D)+g*

(((E+G)−(F+H))/(E+F+G+H)+((I+K)−(J−L))/(I+J+K+L)),  (6)

[0057] The normalization likewise compensates the differences in theintensities of the quantities of light reflected from the three tracksand also the total intensity of the scanning beam directed at theoptical recording medium. The amplitudes of the focus error signalsgenerated from the three beams will thus be of the same size afternormalization. This applies both to the track-position-dependent focusoffset components and to the actual focus error components. In order tomake the track-position-dependent focus offset components equal to zero,the following relationship must therefore hold true:

DFEN _(o) =CFEN _(o) +g*OFEN _(o)=0; OFEN _(o)=2*CFEN ₀  (7)

[0058] In this case, the index “o” designates the respective focusoffset components of the normalized signals DFEN, CFEN and OFEN. Thus,with g=0.5, complete compensation of the track-position-dependent focusoffset component is achieved. In this case, the total focus errorcomponent will be twice as large as that of the primary beam alone.

[0059] A corresponding circuit arrangement for generating thisnormalized offset-compensated focus error signal DFEN is illustrated inFIG. 1. As can be seen from FIG. 1, firstly a normalized CEFN signal anda normalized OFEN signal are generated in accordance with the aboveformulae (4) and (5), the OFEN signal being obtained from twointermediate signals OFE1 and OFE2. The offset-compensated focus errorsignal DFEN is obtained by additive combination of these two normalizederror signals CFEN and OFEN with g=0.5.

[0060] A conventional circuit arrangement for generating the focus errorsignal DFE in accordance with the above formulae (1)-(3) is illustratedfor comparison in FIG. 9.

[0061] Since, in the previously described optical arrangement, thesecondary beams are symmetrical with respect to the primary beam andilluminate the complementary tracks with respect to the tracks detectedby the primary beam, their respective components for forming the actualfocus error signal and the track-position-dependent focus offset isidentical in terms of magnitude. Therefore, the following also holdstrue:

OFE=(((E+G)−(F+H))+((I+K)−(J+L)))*(L′+R′)  (8)

[0062] The sum (L′+R′) is once again proportional to the total quantityof light falling onto the two detectors 10 and 12. Therefore, thenormalization is also valid for both secondary beam components together,so that the following holds true:

OFEN=(((E+G)−(F+H))+((I+K)−(J+L)))/(E+F+G+H+I+J+K+L)  (9)

[0063] It is apparent from this, for a second exemplary embodiment ofthe present invention, whose circuit arrangement is illustrated in FIG.2, that only two dividers have to be used in order to normalize therespective components of the three beams in accordance with thefollowing relationship:

DFEN=((A+C)−(B+D))/(A+B+C+D)+g*

(((E+G)−(F+H))+((I+K)−(J+L)))/(E+F+G+H+I+J+K+L)  (10)

[0064] As a result of the normalization, the intensities of the lightdirected at the three tracks are likewise concomitantly normalized, butby joint normalization in the case of the secondary beams. Theamplitudes of the error signals generated from the two contributionswill thus be of the same size after normalization. This applies both tothe track-position-dependent focus offset components and to the focuserror components. In order to make the focus offset components equal tozero, the following must therefore hold true for g:

DFEN _(o) =CFEN _(o) +g*OFEN _(o)=0; OFEN _(o) =DFEN ₀  (11)

[0065] Consequently, with g=1, complete compensation of thetrack-position-dependent focus offset component is achieved. In thiscase, the total focus error component will be twice as large as that ofthe primary beam.

[0066] The above description makes it clear that, with the aid of thepresent invention, the individual contributions which enter into theoffset-compensated focus error signal can be made independent of thereflection properties of the respectively scanned tracks of the opticalrecording medium.

[0067] Likewise, independence from the instantaneous reflection of therespectively scanned track can be achieved during the generation of anoffset-compensated track error signal in accordance with the DPP methodby means of normalization of the track error components respectivelyread.

[0068] The conventional approach for generating the offset-compensatedDPP signal according to the prior art is as follows:

DPP=CPP−k*OPP  (12)

[0069] In this case, the CPP signal designates the track error signalgenerated in a manner dependent on the reflected primary beam, while theOPP signal represents the track error signal obtained in a mannerdependent on the reflected secondary beams. k denotes the weightingfactor for the weighted combination of the CPP signal and of the OPPsignal.

[0070] The CPP signal and the OPP signal can be expressed as a functionof the reflection factors of the respectively scanned track of theoptical recording medium, assuming the photodetector structure shown inFIG. 10, as follows:

CPP=H′*((A+D)−(B+C))  (13)

OPP=L′*(E 2 −E 1)+R′*(F 2 −F 1)  (14)

[0071] In this case, H′ denotes the reflection factor of the trackscanned by the primary beam, while L′ and R′ denote the reflectionfactors of the tracks scanned by the secondary beams to the left andright of the primary track scanned by the primary beam. As is shown inFIG. 10, a four-quadrant photodetector having photodetector elements A-Dis used for detecting the primary beam, while respective photodetectors10 and 12 having only two photodetector elements E1 and E2, andrespectively F1 and F2, are used for detecting the reflected secondarybeams.

[0072] The positions of the secondary beams on the optical recordingmedium are chosen in such a way that the trace-error-proportionalcomponents of the CPP signal and of the OPP signal are in antiphase.Those components of the CPP signal and of the OPP signal which arecaused by the movement of the objective lens from the optical axis, i.e.the lens-movement-proportional components, are in phase, however. If thefactor k is chosen correctly, then these lens-movement-proportionalcomponents of the CPP signal and of the OPP signal are mutuallycancelled out during subtraction. Therefore, the factor k is to bechosen in such a way that the following holds true:

DPP _(l) =CPP _(l) −k*OPP _(l)=0  (15)

[0073] The index “l” denotes the lens-movement-proportional orlens-movement-dependent component of the individual signals.

[0074] The value for the weighting factor k can be defined only if thereflection factors H′, L′ and R′ are identical or constant. As hasalready been described, however, this is not always ensured. Accordingto the invention, however, independence of the weighting factor k fromthe different reflection properties of the respectively scanned tracksof the optical recording medium is achieved by means of normalization.

[0075] As has already been described, the reflection factor H′ of theprimary beam is proportional to the total quantity of light whichstrikes the photodetector 11 having the photodetector elements A-D.Consequently, analogously to the previously described case ofnormalization of the DFE signal, by division by the summation signal ofthe individual photodetector elements A-D, normalization for the primarybeam can be achieved as follows:

CPPN=((A+D)−(B+C))/(A+B+C+D)  (16)

[0076] In this case, CPPN denotes the normalized CPP signal. Anormalized OPP signal OPPN can likewise be defined for the reflectionfactors L′ and R′:

OPPN=(E 2 −E 1)/(E 1+E 2)+(F 2−F 1)/(F 1+F 2)  (17)

[0077] Consequently, from the formulae (12), (16) and (17), therefollows for the generation of a normalized DPP signal DPPN:

DPPN=((A+D)−(B+C))/(A+B+C+D)−

k*((E 2−E 1)/(E 1+E 2)+(F 2−F 1)/(F 1 +F 2))  (18)

[0078] The normalization likewise concomitantly normalizes thedifferences in the intensities of the light directed at the threetracks. The amplitudes of the error signals generated from the threebeams will thus be of the same size after normalization. From theformula (15), it follows for the lens-movement-dependent component where

OPP _(l)=2*CPP _(l),  (19)

[0079] that, for k=0.5, complete compensation of thelens-movement-dependent component can be achieved. In this case, thetrack-error-dependent component will be twice as large as that of theprimary beam alone.

[0080]FIG. 3 illustrates an exemplary embodiment—corresponding to theformula (18)—for a circuit arrangement according to the invention forgenerating the offset-compensated normalized track error signal DPPN,three dividers being used separately in order to normalize theindividual beams.

[0081] A conventional circuit arrangement according to the prior art forgenerating the track error signal DPP in accordance with the aboveformulae (12)-(14) is illustrated for comparison in FIG. 11. As can beseen from FIG. 11, the CPP and OPP signals are not normalized inaccordance with the prior art. Therefore, it is necessary for theweighting factor k to be continuously adapted to the reflectionproperties of the respectively scanned tracks.

[0082] Since, in the case of the previously mentioned opticalarrangement, the secondary beams are arranged symmetrically with respectto the primary beam, their respective components for forming the trackerror signal has the same magnitude. The following therefore holds true:

OPP=((E 2+F 2)−(E 1 +F 1))*(L′+R′)  (20)

[0083] The sum (L′+R′) is once again proportional to the total quantityof light falling onto the detector elements E1, E2, F1 and F2.Therefore, the following normalization valid for both secondary beamcomponents can be carried out:

OPPN=((E 2 +F 2)−(E 1 +F 1))/(E 1 +E 2 +F 1 +F 2)  (21)

[0084] The following emerges from this for the normalized error signalDPPN:

DPPN=((A+D)−(B+C))/(A+B+C+D)

k*(((E 2 +F 2)−(E 1 +F 1))/(E 1 +E 2 +F 1 +F 2))  (22)

[0085] The normalization likewise concomitantly normalizes thedifferences in the intensities of the light directed at the threetracks. The amplitudes of the error signals generated from the threebeams will thus be of the same size after normalization. For completecompensation of the lens-movement-dependent component, the followingrelationship holds true:

DPP ₁ =CPP ₁ −k*OPP _(l)=0; OPP _(l) =CPP ₁  (23)

[0086] Consequently, with k=1, complete compensation of thelens-movement-dependent component of the track error signal DPPN can beachieved. In this case, the track-error-dependent component is twice aslarge as that of the primary beam alone.

[0087] A corresponding circuit arrangement for generating the correctedor compensated track error signal DPPN in accordance with the aboveformula (22) is illustrated in FIG. 4. As can be seen from FIG. 4, onlytwo dividers are required for generating the normalized signal CPPN orthe normalized signal OPPN, which are subsequently combined by weightedsubtraction with k=1 to form the compensated track error signal DPPN.

[0088] It goes without saying that the above DPP method can also beemployed if the photodetectors in each case have four light-sensitiveareas. In this case, corresponding summation signals are still formed bytwo respective detector areas.

[0089] Furthermore, it should be noted that the previously describedweighting factors g and k are valid only when component tolerances andother tolerances do not have to be taken into account. By way ofexample, an error source might be the normalization elements used inFIGS. 1-4, since divisions are difficult to realize using analoguetechnology. Consequently, the previously described weighting factorsapply only to the ideal case. In order to compensate componenttolerances, if appropriate a departure is made from these values to agreater or lesser extent.

[0090] Likewise, it is possible, in contrast to the exemplaryembodiments illustrated in FIGS. 1-4, to apply the weighting factors gand k to the primary beam signals CFEN and CPPN, respectively, as well,so that the normalized focus error signal DFEN is calculated inaccordance with the following formula:

DFEN=g′*CFEN+OFEN where g′=1/g  (24)

[0091] The normalized track error signal DPPN is then analogouslycalculated in accordance with the following formula:

DPPN=k′*CPPN−OPPN where k′=1/k  (25)

[0092] In the case of the previously described exemplary embodimentsillustrated in FIGS. 1-4, the normalization was in each case employedseparately in order to form a focus or track error signal. However, thecircuitry outlay can be reduced if the normalization is employed forforming both the focus and the track error signal, since then thesummation for obtaining the respective normalization signal can be usedjointly for both signal paths. FIG. 12 and FIG. 13 illustratecorresponding exemplary embodiments, FIG. 12 showing an exemplaryembodiment corresponding to FIGS. 1 and 3 in which the secondaryscanning beams are normalized separately in each case (weighting factorsg, k=0.5), while FIG. 13 shows an exemplary embodiment corresponding toFIGS. 2 and 4 with joint normalization of the secondary scanning beams(weighting factors g, k=1).

[0093] The illustrations of FIG. 12 and FIG. 13 also reveal how the DPPmethod can be applied to three photodetectors 10-12 each having fourlight-sensitive areas. In this case, the two photodetector elements E1,E2 and F1, F2 of the embodiments described above correspond to thephotodetector elements F and G, E and H and, respectively J and K, I andL.

[0094] Consequently, according to the invention, a corrected orcompensated focus error signal DFEN or track error signal DPPN isobtained by primary and secondary scanning beams incident on adjacenttracks of an optical recording medium 7 being generated and the primaryand secondary scanning beams reflected from the optical recording mediumbeing detected in order to derive therefrom primary-beam andsecondary-beam focus error signals CFE, OFE or primary-beam andsecondary-beam track error signals CPP, OPP, which are subsequentlynormalized in order to obtain the compensated focus error signal DFENerror track or signal DPPN from the normalized primary-beam andsecondary-beam error signals CFEN, OFEN; CPPN, OPPN by means of weightedcombination. As a result of the normalization, the corrected orcompensated circuit error signal DFEN or track error signal DPPN can begenerated independently of the reflection properties of the respectivelyscanned track.

[0095] An apparatus according to the invention is suitable for readingfrom and/or writing to optical recording media which have, in terms oftheir physical properties, different track types arranged adjacent toone another. The apparatus has a beam generation unit for generatingprimary and secondary scanning beams incident on adjacent tracks of arecording medium, a photodetector having multi-zone detector elementsfor detecting the primary and secondary scanning beams reflected fromthe recording medium, and an evaluation circuit for forming a correctederror signal by weighted combination of primary-beam and secondary-beamerror signals formed from the detected signals of the primary andsecondary scanning beams. In this case, the evaluation circuit hasnormalizing means for normalizing primary-beam and secondary-beam errorsignals.

[0096] As already mentioned, for all the above considerations it wasassumed in a simplification that the intensities of the three scanningbeams considered are identical when impinging on the photodetector unit9. Therefore, the compensation factors g and k specified apply only ifthis simplification is employed.

[0097] In practice, however, the intensity of the secondary beams isdependent on their track position, on the reflection of the scannedtrack and also on the properties of the optical diffraction grating 3and is weaker than the intensity of the primary beam, so that theintensity of the secondary beams must be scaled correspondingly withrespect to the primary beam intensity. Ideally, this is done bynormalization. To that end, the signals derived from the reflected beamsare normalized. The signals CPP and OPP or, alternatively, theindividual signals OPP1 and OPP2 are normalized by these signals beingdivided by the summation signals which are proportional to the quantityof light respectively taken up by the detector areas.

[0098] As described above, it is necessary to adapt the weighting factorg or k to the secondary track spacings. By way of example, if thevariant shown in FIG. 3 is taken as a basis, then the signal amplitudesof the signal DPPN is dependent on the setting of the compensationfactor k.

[0099] The variant of the embodiments in accordance with FIGS. 3 and 4which is shown in FIGS. 14 and 15, respectively, relates to theweighting between primary beam and secondary beams. By way of example,the weighting factor k for the secondary beam signal is advantageouslyreplaced by two weighting factors k′ and 1−k′ which act on the primaryand secondary beam signals, where k′ can be calculated from k accordingto the following relationship:$k^{\prime} = \frac{k}{( {1 + k} )}$

[0100] The effects achieved by splitting the weighting factor k into twoweighting factors dependent on k′ is that the amplitude of thenormalized signal DPPN is independent of the weighting factor to be setin each case. Correspondingly, this formula can also be applied to theweighting factor g for forming the signal DFEN. The factors g and k arechosen for example in the manner described with respect to FIG. 3 andFIG. 4.

1. Method for generating a corrected error signal for the operation ofan apparatus for reading from and/or writing to an optical recordingmedium, primary and secondary scanning beams (13-15) incident onadjacent tracks of the recording medium (7) being generated and theprimary and secondary scanning beams reflected from the recording medium(7) being detected, and primary-beam and secondary-beam error signals(CFE, OFE; CPP, OPP) being derived from the detected reflected primaryand secondary scanning beams and being combined with one another in aweighted manner in order to form a corrected error signal, characterizedin that the primary-beam and secondary-beam error signals (CFE, OFE;CPP, OPP) are normalized before the corrected error signal (DFEN; DPPN)is formed by weighted combination therefrom.
 2. Method according toclaim 1, characterized in that the primary-beam error signal (CFE; CPP)and the secondary-beam error signals (OFE; OPP) are in each casenormalized separately.
 3. Method according to claim 1, characterized inthat the secondary-beam error signals (OFE; OPP) are normalizedtogether.
 4. Method according to one of the preceding claims,characterized in that the primary-beam and secondary-beam error signals(CFE, OFE) are focus error signals which are normalized in ordersubsequently to obtain a corrected focus error signal (DFEN) by means ofweighted combination.
 5. Method according to claim 4, characterized inthat the corrected focus error signal DFEN is obtained from thenormalized primary-beam focus error signal CFEN and the normalizedsecondary-beam focus error signal OFEN in accordance with the followingrelationship: DFEN=CFEN+g*OFEN, where g denotes a weighting factor. 6.Method according to claims 2 and 5, characterized in that a primaryscanning beam (14) and two secondary scanning beams (13, 15) aregenerated and the primary and secondary scanning beams reflected fromthe optical recording medium (7) are detected by photodetectors (10-12)each having four photodetector elements, and in that the corrected focuserror signal DFEN is obtained in accordance with the followingrelationship: DFEN=((A+C)−(B+D))/(A+B+C+D)+g*(((E+G)−(F+H))/(E+F+G+H)+((I+K)−(J−L))/(I+J+K+L)), where A-D denotes theoutput signals of the photodetector elements of the photodetector (11)which detects the reflected primary scanning beam, while E-H and I-Ldenote the output signals of the photodetector elements of thephotodetectors (10, 12) which detect the reflected secondary scanningbeams.
 7. Method according to claim 6, characterized in that g=0.5 ischosen for the weighting factor.
 8. Method according to claims 3 and 5,characterized in that a primary scanning beam (14) and two secondaryscanning beams (13, 15) are generated and the primary and secondaryscanning beams reflected from the optical recording medium (7) aredetected by photodetectors (10-12) each having four photodetectorelements, and in that the corrected focus error signal DFEN is obtainedin accordance with the following relationship:DFEN=((A+C)−(B+D))/(A+B+C+D)+g*(((E+G)−(F+H))+((I+K)−(J+L)))/(E+F+G+H+I+J+K+L), where A-D denote theoutput signals of the photodetector elements of the photodetector (11)which detects the reflected primary scanning beam, while E-H and I-Ldenote the output signals of the photodetector elements of thephotodetectors (10, 12) which detect the reflected secondary scanningbeams.
 9. Method according to claim 8, characterized in that g=1 ischosen for the weighting factor.
 10. Method according to one of claims1-3, characterized in that the primary-beam and secondary-beam errorsignals (CPP, OPP) are track error signals which are normalized in orderto obtain a corrected track error signal (DPPN) by means of weightedcombination.
 11. Method according to claim 10, characterized in that thecorrected track error signal DPPN is obtained from the normalizedprimary-beam track error signal CPPN and the normalized secondary-beamerror signal OPPN in accordance with the following relationship:DPPN=CPPN−k*OPPN, where k denotes a weighting factor.
 12. Methodaccording to claims 2 and 11, characterized in that a primary scanningbeam (14) and two secondary scanning beams (13, 15) are generated andthe secondary scanning beams reflected from the optical recording medium(7) are each detected by photodetectors (10, 12) having twophotodetector elements, while the primary scanning beam reflected fromthe optical recording medium (7) is detected by a photodetector (11)having four photodetector elements, and in that the corrected trackerror signal DPPN is obtained in accordance with the followingrelationship: DPPN=((A+D)−(B+C))/(A+B+C+D) k*((E 2 −E 1)/(E 1 +E 2)+(F 2−F 1)/(F 1 +F 2)), where A-D denotes the output signals of thephotodetector elements of the photodetector (11) provided for detectionof the reflected primary scanning beam, while E1 and E2, andrespectively F1 and F2, denote the output signals of the photodetectorelements of the photodetectors (10, 12) provided for detection of thereflected secondary scanning beams.
 13. Method according to claim 12,characterized in that k=0.5 is chosen for the weighting factor. 14.Method according to claims 3 and 11, characterized in that a primaryscanning beam (14) and two secondary scanning beams (13, 15) aregenerated and the secondary scanning beams reflected from the opticalrecording medium (7) are each detected by photodetectors (10, 12) havingtwo photodetector elements, while the primary scanning beam reflectedfrom the optical recording medium (7) is detected by a photodetector(11) having four photodetector elements, and in that the corrected trackerror signal DPPN is obtained in accordance with the followingrelationship: DPPN=((A+D)−(B+C))/(A+B+C+D)− k*(((E 2 +F 2)−(E 1 +F1))/(E 1 +E 2 +F 1 +F 2)), where A-D denotes the output signals of thephotodetector elements of the photodetector (11) provided for detectionof the reflected primary scanning beam, while E1 and E2, andrespectively F1 and F2, denote the output signals of the photodetectorelements of the photodetectors (10, 12) provided for detection of thereflected secondary scanning beams.
 15. Method according to claim 14,characterized in that k=1 is chosen for the weighting factor. 16.Apparatus for reading from and/or writing to an optical recordingmedium, having a beam generation unit (1-3) for generating primary andsecondary scanning beams (13-15) incident on adjacent tracks of theoptical recording medium (7), having a photodetector unit (9) fordetecting the primary and secondary scanning beams reflected from theoptical recording medium (7) and having an evaluation unit (16) forforming a corrected error signal by weighted combination of primary-beamand secondary-beam error signals (CFE, OFE; CPP, OPP) derived from thedetected reflected primary and secondary scanning beams, characterizedin that the evaluation unit (16) has normalization means for normalizingthe primary-beam and secondary-beam error signals (CFE, OFE; CPP, OPP)before the weighted combination thereof to form the corrected errorsignal (DFEN; DPPN).
 17. Apparatus according to claim 16, characterizedin that the normalization means are configured for in each case separatenormalization of the primary-beam error signal (CFE; CPP) and of thesecondary-beam error signals (OFE; OPP).
 18. Apparatus according toclaim 16, characterized in that the normalization means are configuredin such a way that the secondary-beam error signals (OFE; OPP) arenormalized together.
 19. Apparatus according to one of claims 16-18,characterized in that the evaluation unit is configured for generatingprimary-beam and secondary-beam focus error signals (CFE, OFE) and forgenerating a corrected focus error signal (DFEN) by weighted combinationof the normalized primary-beam and secondary-beam focus error signals(DFEN, OFEN).
 20. Apparatus according to claim 19, characterized in thatthe evaluation unit (16) or the normalization means are configured forcarrying out a method according to one of claims 5-9.
 21. Apparatusaccording to one of claims 16-18, characterized in that the evaluationunit (16) is configured for generating primary-beam and secondary-beamtrack error signals (CPP, OPP) and for generating a corrected trackerror signal (DPPN) by weighted combination of the normalizedprimary-beam and secondary-beam track error signals (CPPN, OPPN). 22.Apparatus according to claim 21, characterized in that the evaluationunit (16) or the normalization means are configured for carrying out amethod according to one of claims 11-15.
 23. Method for generating acorrected error signal for the operation of an apparatus for readingfrom and/or writing to an optical recording medium, primary andsecondary scanning beams (13-15) incident on adjacent tracks of therecording medium (7) being generated and the primary and secondaryscanning beams reflected from the recording medium (7) being detected,and primary-beam and secondary-beam error signals (CFE, OFE; CPP, OPP)being derived from the detected reflected primary and secondary scanningbeams and being combined with one another in a weighted manner in orderto form a corrected error signal, characterized in that the primary-beamand secondary-beam error signals (CFE, OFE; CPP, OPP) are normalizedbefore the corrected error signal (DFEN; DPPN) is formed by weightedcombination therefrom wherein normalisation is performed using a signal(A+B+C+D, E+F+G+H, I+J+K+L, E+F+G+H+I+J+K+L, E1+E2, F1+F2, E1+E2+F1+F2)corresponding to the intensity of the reflected primary-beam (14) orsecondary-beam (13,15), respectively.
 24. Apparatus for reading fromand/or writing to an optical recording medium, having a beam generationunit (1-3) for generating primary and secondary scanning beams (13-15)incident on adjacent tracks of the optical recording medium (7), havinga photodetector unit (9) for detecting the primary and secondaryscanning beams reflected from the optical recording medium (7) andhaving an evaluation unit (16) for forming a corrected error signal byweighted combination of primary-beam and secondary-beam error signals(CFE, OFE; CPP, OPP) derived from the detected reflected primary andsecondary scanning beams, characterized in that the evaluation unit (16)has normalization means for normalizing the primary-beam andsecondary-beam error signals (CFE, OFE; CPP, OPP) before the weightedcombination thereof to form the corrected error signal (DFEN; DPPN),wherein normalisation is performed using a signal (A+B+C+D, E+F+G+H,I+J+K+L, E+F+G+H+I+J+K+L, E1+E2, F1+F2, E1+E2+F1+F2) corresponding tothe intensity of the reflected primary-beam (14) or secondary-beam(13,15), respectively.